The present invention relates to a method for the synthesis of metalloaluminophosphate (MeAPO) molecular sieves and particularly to a process for the synthesis of silicoaluminophosphate (SAPO) molecular sieves.
Methods for the preparation of metalloaluminophosphates, such as MeAPSOs, EIAPSOs, and MeAPOs are known in the art and are members of a class known as non-zeolitic molecular sieves. (See E. M. Flanigen, B. M. Lok, R. L. Patton, S. T. Wilson, New Developments in Zeolite Science and Technology, Y. Murakemi et al. eds., Elsevier, Amsterdam, p. 103(1986)). These non-zeolitic molecular sieves are referred to herein as MeAPO molecular sieves and have frameworks of AlO4, MeOx and PO4 tetrahedra linked by oxygen atoms. A negative change in the framework is balanced by the inclusion of exchangeable protons or cations such as alkali or alkaline earth metal ions. The interstitial spaces of channels formed by the crystalline framework enables MeAPOs to be used as molecular sieves in a manner similar to aluminosilicate molecular sieves, such as, for example, zeolites.
Accordingly, numerous microporous framework structures analogous to the aluminosilicate zeolites can be synthesized having an AlPO4 composition and have been called ALPOs. A modified family of materials has been made by the substitution of a metal (Me) for Al3+ and P5+ to form the above referenced metals. Although the ALPO structures are neutral frameworks, the substitution of Ma+ for P5+ imparts a negative charge on the framework when xe2x80x98axe2x80x99 is less than 5. By suitable choice of a cation, this can be translated into catalytic activity. However, alternate substitutions may be possible that may result in a disproportionately low exchange capacity. The exact nature of Me substitution into ALPO structures is complex and highly variable and may depend on both the topology of the ALPO/MeAPO and the method of preparation. (See D. Barthomeuf, J. Phys. Chem., 97, p. 10092 (1993) and D. Barthomeuf, Zeolites, 14, p. 394 (1994)). The result is that preferred catalysts may be made by a suitable choice of synthesis method.
Because the distribution of Me in the MeAPO framework affects catalytic activity, the catalytic activity of MeAPOs therefore depends on both the global composition and the Me distribution. This has been bet demonstrated for Si4+ on the basis of this NMR techniques. It has been shown for some substitutions that when MeAPOs contain low amounts of the Me, the metal atoms are mostly isolated. However, when the Me content increases, islands of the metal may start to appear, i.e., Me sites having metal atoms and no aluminum or phosphorus atoms in neighboring lattice positions. These metal islands result in a loss of catalytic activity of the molecular sieve, and increase the cost of the catalyst when using expensive metals such as gallium and germanium.
It would, therefore, be desirable to further increase metal content in a MeAPO molecular sieve without forming undesirable metal islands in order to increase catalytic activity and selectivity.
In order to overcome many of the problems inherent in the prior art, the present invention provides a method for the synthesis of MeAPO molecular sieves.
The method includes the following steps: providing a source of alumina, a source of phosphorus, water, and a template suitable for forming a MeAPO molecular sieve; providing a source of metal (Me) including metal particles, the metal particles measuring, in their largest dimension, equal to or less than five nanometers; providing a water soluble organic solvent capable of solubilizing the source of metal; forming a synthesis mixture from the source of alumina, the source of phosphorus, the water, the template, the source of metal, and the solvent; and forming a MeAPO molecular sieve from the synthesis mixture. Desirably, the metal is silicon and the MeAPO is a silicoaluminophosphate molecular sieve.
Another embodiment of the present invention is directed to an isocrystalline spheroidal particle comprising a SAPO molecular sieve. The particle measures from 0.5 microns to 30 microns in diameter. The particle further includes crystallites measuring from 0.05 microns to 1 micron at their largest dimension. This particle is desirably made by the process set forth above. Desirably, the particle is a SAPO-34 molecular sieve.
Other advantages and uses of the present invention will become apparent from the following detailed description, appended figures, and appended claims.